1. Field of the Invention
The present invention relates to a fluid forcing nozzle. More particularly the invention relates to a fluid forcing nozzle of the type which may alternately inject a fluid or a gas into the interior of a mold cavity. The construction disclosed is greatly simplified in comparison to prior art nozzles, and uses a minimum of moving parts.
2. Description of the Prior Art
The use of nozzles to force fluids and/or gases into the interiors of various types of cavities is well known in the art. One such use of fluid forcing nozzles is in the field of injection molding, and more particularly in the field of gas assisted injection molding. Gas assisted injection molding is known in the art as a preferred way of making larger and more complicated parts without having the problems of sink marks and the like in thick rib sections or when complicated cross sections are involved.
In this type of injection molding, these large and complicated pieces are made of a hollow construction by injecting a fluid or gas into the interior of a part while it is being formed in a mold cavity. It is known that the gas or fluid which is injected into the interior of the mold cavity will follow the easiest path. Since it is also known that the molten plastic or synthetic resin will generally cool and harden from the mold cavity wall inwardly, the interior of the part will stay fluid longer, and the gas will penetrate the sorer and warmer portions of the part. The softer and warmer portions of the part are generally found in the thicker sections where there is a structural rib, etc. The injection gas, by following this path, creates a hollow within the plastic, and that hollow can be expanded by the addition of pressure outwardly to fill the mold cavity.
If the gas pressure is held at an effectively constant level while the plastic part cools, the surface finish of the plastic part will be greatly enhanced by the elimination of sink marks, etc. Along with the idea of the injection of the gas or fluid into the interior of the mold cavity through the fluid forcing nozzle came the need to vent the gas to relieve the pressure within the part before the mold could be opened. Failure to relieve this pressure, in some cases, caused the part to explode when the mold opened. Many solutions were tried to discover a satisfactory way of introducing the gas into the interior of the plastic part, and then to vent the gas before opening the mold cavity.
An apparatus for venting the gas after injecting the fluid through the fluid forcing nozzle, and before opening the mold cavity, is described in U.S. Pat. No. 4,943,407 issued Jul. 24, 1990 to James W. Hendry. Generally this invention involves venting the gas to atmosphere through a specialized sprue bushing in which the sprue bustling includes a first body part in which a second body part or cylindrical pin is mounted for movement upon the activation of an actuator. The pin has two flow paths which are alternately used. One path allows the passage of plastic through the bushing into the sprue, and from there into the interior of the mold cavity. The second path allows injection or venting of gas from the interior of said mold cavity.
However, by creating the two f low paths in the mold sprue bushing, one is required to provide the special sprue bushing valves for each mold being used. When molds having multiple cavities are used, this increases the cost of the mold significantly. In addition, mold sprue bushings are generally rather small, and, therefore, providing moving parts therein to allow the injection of plastic and gas alternately has proven difficult.
Applicant's assignee, to eliminate the problems in the prior art, developed the idea of forcing fluid into the interior of the mold cavity through the nozzle, which is a larger device, can be more easily manufactured, and can serve a large number of mold cavities. One such nozzle is described in co-pending U.S. patent application Ser. No. 07/628,746, filed on Dec. 17, 1990, now U.S. Pat. No. 5,200,127 and assigned to the common assignee of the present invention.
Another such nozzle is described in a co-pending application by the inventor of the present application filed on every date herewith entitled "INJECTION NOZZLE" and assigned to the common assignee of the present invention. These nozzles have proven to be satisfactory for many applications. However, for some applications it was desired to develop a nozzle with an absolute minimum of moving parts to prevent problems known in the art of injection molding.